« SebelumnyaLanjutkan »
its surface, but the adjacent country also. As the part crossed was nearly a mile in breadth, many opportunities presented themselves of remarking the temperatures of the water and the mist. The river was uniformly 530 all over its surface; on the eastern shore, and for three hundred yards across the temperature of the air was 42° ; as the middle of the river was approached, it subsided to 41°, and again gradually rose until, on the western bank, it had arrived to 43°; the air in the fields close to the river was 42°.
A current of dry air, however, appears to have the effect of suspending all these operations, even where the relative conditions of the water and the air might be supposed to be very favorable to them. Sir H. Davy, during his Danube voyage, observed, that during a strong easterly wind do mists were formed, when the water was 61°, and the air but 54°; a re. markable contrast to the case of the rivers Inn and Ilz before mentioned, where the difference of a degree or two only was marked by deposition. Mr. Harvey repeatedly observed this effect of dry air at Plymouth.
The peculiar odor which prevails in some fogs, has never yet been satisfactorily accounted for.
RAIN, HAIL, AND SNOW. The water which we see descending from the clouds, in the fluid state of drops, or crystallized in snow, or congealed in hail, is in the final stage of that perpetually recurring journey, which some portion of this element is constantly performing through the atmosphere of our planet.
Lifted up, perhaps, from the surface of soine remote ocean, by the process of evaporation, into the region of the winds, and subjected to a play of temperature infinitely varied, it may, by the changes of its locality, have presented its beautiful evanescent apparition to all the nations of the earth, before that junction of coincidences happened, which produced its fall at
This last change, which preceded and terminated its aërial career, was but another effect of the same extensively operating cause which produces, in the transparent atmosphere, the deposition of those minute particles of water which constitute a cloud, and which, when greatly increased in energy, assembles and pours them in rain.
“ The profuse precipitation of humidity which has received this appellation, is caused by a rapid commixture of opposite strata of air of different temperatures. The action of swift contending currents in the atmosphere, brings quickly into mutual contact vast fields of air over a given spot. The separation of moisture is proportionally rapid and copious; the particles conglomerate, and in temperate weather the deposition descends to the earth in the form of rain. In the cold season the aqueous globules, freezing in the mid air into icy spiculæ, collect together during their descent, and become converted, ere they reach the earth's surface, into flakes of snow. Hail is formed under different circumstances, and generally in sudden altercations of the fine season, the globules of rain being
congealed during their fall, by passing through a stratum of dry and cold air.
“ The drops of rain vary in their size, perhaps from the 25th to the 4th of an inch in diameter. In parting from the clouds, they precipitate their descent till the increasing resistance opposed by the air, becomes equal to their weight, when they continue to fall with a uniform velocity. This velocity is, therefore, in a certain ratio, to the diameter of the drops ; hence thunder, and other showers in which the drops are large, pour down faster than a drizzling rain. A drop of the 25th part of an inch, in falling through the air, would, when it had arrived at its uniform velocity, only acquire a celerity of 114 feet per second; while one of 4th of an inch would acquire a velocity of 33} feet.
“ A flake of snow being, perhaps, nine times more expanded than water, would descend thrice as slow. Hail-stones are often of considerale dimensions, exceeding sometimes the length of an inch; they may, therefore, fall with a velocity of 70 feet per second, or at the rate of about fifty miles in the hour. Striking the ground with such impetuous force, it is easy to conceive the extensive injury which a hail shower may occasion in the hotter climates. The destructive power of these missiles in stripping and tearing the fruits and foliage, increases besides in a faster ratio than the momentum, and may be estimated by the square of their velocity multiplied into their mass. This fatal energy is hence as the fourth power of the diameter of the hail-stone."* The celebrated Volta referred the formation of hail to the play of electricity among the clouds, and upon this theory proposed the erection of paragrêles, or hail-rods, in countries much exposed to the ravages of hail-storms. These, upon the same principle as lightning rods, were to consist of lofty poles tipped with metallic points, and having metallic wires communicating with the earth. By thus subtracting superabundant electricity from clouds, Volta imagined that the formation of hail would be prevented. These paragrêles have been tried in Switzerland upon an extensive scale; but their success has not been proportionate to the expectations which were formed from a minute experiment.
The average quantity of rain which falls in the course of a year, in the neighbourhood of London, according to Mr. Daniell's observations, amounts to 23 1'ó inches, or, if collected, it would form a sheet of water of that depth. The registers of the rain which have been kept in various parts of the United Kingdom, have given results which have excited some doubt of their accuracy. Mr. Leslie thinks, that " in general twice as much rain falls on the western as on the eastern side of our island, and that the average annual quantity may be reckoned at 30 inches. According to this estimate, the whole discharge from the clouds in the course of a year, on
every square mile of the surface of Great Britain, would, at a medium, be 1,944,633, or nearly two millions of tons. This gives about three thousand tons of water for each English acre, a quantity equal to 630,000 imperial gallons.”
The contributions of the several months to the production of this quantity, are stated by Mr. Daniell, and recorded in the British Almanac, to be in the following proportions :January 1.483 July
1.440 September 2.193
2.426 The greatest average quantity, therefore, falls in July, and the smallest in February
In comparing quantities which fall in the twenty-four hours, which constitute a day, the result of experiment shows that a greater amount of rain falls while the sun is below, than when above the horizon,
One very remarkable circumstance attending the fall of rain, is, “ that smaller quantities have been observed to be deposited in high than in low situations, even though the difference of altitude should be inconsiderable. Similar observations have been made at the summit, and near the base of hills of no great elevation. Rain-gauges, placed on both sides of a hill at the bottom, always indicate a greater fall of rain than on the exposed top.”*
If the whole of the waters which fall from the heavens were to return again, the evaporation from the ground might be sufficient alone to inaintain the perpetual circulation. But more than one-third of all the rains and snows are carried by the rivers into the ocean, which must hence restore this continued waste.
The Natural History of the Weather embraces the phenomena of Dew and Hoar Frost, Dryness and Moisture, Heat and Cold, Thunder and Lightning, and Winds : as well as those of Evaporation, Clouds, Fogs and Mist, and Rain, to which the present article is necessarily limited by our space. The subject will be pursued in the Almanac for 1832.
II. SHAPE OF THE EARTH, AND ITS SIZE.
The level portions of the earth's surface seem at first view perfectly flat. But if we examine them more critically, and for a considerable extent, we shall find that they are decidedly convex, or swelled out in the middle. The light of a light-house requires to be raised, in order to be seen at any considerable distance. Let it be placed on a level with the sea, and a person of the common height, or whose eyes are less than six feet above the surface of the sea, would not be able to see it at the distance of four miles, however strong and clear the light might be. But upon raising himself higher and higher, he would at length, when his eye had reached an elevation of ten or eleven feet above the surface, be able to discern it just grazing the surface of the water. The same effect would be produced if the light were raised ten or eleven feet, and the eye of the observer were on the level of the ocean. And a light 60 or 100 feet high disappears in like manner by sinking lower and lower; only the distance at which we are required to place ourselves to produce this effect, becomes greater and greater according to the elevation of the light, and according also to our own elevation above the level of the sea. The most convenient position for a nice observation of this kind is an extended lake, when covered with smooth ice. We will suppose ourselves provided with a common leveling instrument, or any long tube capable of being fixed in an exactly horizontal position, which is easily determined by a water-level, or by being at right angles to a plumb-line. Let us suppose that the line of sight through the tube is precisely four feet from the ice, and that the tube can be turned in all directions without varying from a horizontal or level position. If we now look through the tube at an upright rod or pole placed with one end on the ice at different distances, we shall be able to establish, in the most satisfactory manner, the following important facts.
1. The line of sight, or apparent level, as it is called, departs from the surface of the ice, or true level, in whatever direction we look.
2 This departure, or difference of level, is the same in all directions as to the points of the compass, where the distance from the observer is the same.
3. The difference of level for a distance of one mile is 8 inches.
4. If we double any distance, the difference of level is quadrupled, and if we triple the distance, the difference of level is nine times as great, and so on, according to the law of the squares ; that is, the difference of level for one mile being 8 inches, that for two miles is not twice 8, but four times 8, or 32 inches, and that for three miles is 9 times 8, or 72 inches.
Similar observations being made in other places in different parts of the earth, we arrive at essentially the same results.
The facts above given, lead to conclusions not less curious and striking.
1. The earth's surface is curved instead of being plane, or flat, and plumblines or lines perpendicular to the surface, are not strictly parallel, but incline more and more the further they are apart, and tend to meet at some point within.
2. The earth appears to be equally curved in all directions, and the law of the departure of the apparent from the true level, indicates a spherical surface.
3. The particular departure of 8 inches to a mile points out the dimensions of the earth, and furnishes, by means of a simple proposition in geometry, a method of calculating its diameter. Thus
A B in the adjoining figure we have AB and BD to find AE, or BE, which does not sensibly differ from AE, since BD, by supposition, is only eight inches. It is a very familiar proposition in plane geometry, that, when from a point without a circle two lines be drawn, one cutting and the other touching it, the touching line is a mean proportional between the cutting line and the part without the circle; hence
BD: AB :: AB: BE or AE very nearly ; that is, 8 inches being 7027 of a mile,
qozo:1::1: 7920; in other words, the earth's diameter is 7920 miles. This is almost precisely what it is fixed at by the most elaborate observations and calculations. As the circumference of a circle or sphere exceeds its diameter a little more than three times (34), if we multiply the above result by 37, we have the circumference equal to 24,890 miles.
The common way of determining the magnitude of the earth, is by measuring a certain part of its circumference in the direction of the meridian. Lake Champlain, for example, when frozen over, would furnish a proper field for such an operation. Two plumb-lines being suspended, on the same meridian, one at Crown Point and the other on the boundary line between the United States and Canada, would be found to deviate from parallelism one degree, that is, they would meet near the centre of the earth, having an inclination, or forming an angle, of one degree, or são part of a circumference, and the distance between these plumblines being actually measured with a chain, would be the 360th part of the entire circuit of the globe. The inclination of the plumb-lines above mentioned, is the same thing as the difference of latitude of the two places, and is found by taking the altitude (or angular distance above the horizon) of the pole by means of the Pole star, or other star in the neighborhood. Portions of the earth's circumference, in various countries and regions, have been determined in this way with the greatest care and exactness, and